(i) Field of the Invention
The present invention relates to an optical storage apparatus in which by radiating light to a storage medium with information stored therein and receiving the light subjected to a polarized state change in the storage medium, the information stored in the storage medium is read based on the polarized state change.
(ii) Description of the Related Art
As one type of the storage medium in which the information is read based on the change of the polarized state as described above, optical disks such as a magnetic optical (MO) disk have been noted, this type of high capacity storage medium is small-sized, light-weighted, and portable, and further possibilities of high densification and capacity enlargement have been pursued.
In a conventional optical disk drive in which the optical disk is accessed, the optical disk is irradiated with light, and the information stored in the optical disk is read by picking up the reflected light whose polarized state is changed in accordance with the information stored in the optical disk. Specifically, the light reflected by the optical disk is split from the light of a forward path to be radiated to the optical disk by a beam splitter, and further separated into both P, S polarized component beams crossing at right angles to each other by a Wollaston prism, and the beams are incident upon and received by two divided light receiving elements. The received light signal is subjected to a pre-processing by an analog circuit, and subsequently to a signal extraction processing, so that the information is read out.
Here, in the reflected light from the optical disk, besides the change of the polarized state by the intrinsic action of the stored information, a phase change is also generated between both P, S polarized components by the birefringence of the protective layer of the optical disk medium, and further the birefringence is nonuniform over the entire surface of the optical disk and also fluctuates even in one track. Furthermore, the optical system for guiding the reflected light from the optical disk to the light receiving element is provided with a polarized beam splitter, a reflective mirror, and other optical elements in which a phase deviation is possibly generated between both P, S polarized components of the reflected light from the optical disk.
This phase deviation between both P, S polarized components appears as the xe2x80x9csurgexe2x80x9d of the DC component of the regenerated signal obtained in the stage of pre-processing by an analog circuit after the light receiving element receives light. Even in the optical system, if the phase deviation is generated, the xe2x80x9csurgexe2x80x9d is remarkably largely emphasized, and there is a possibility that the range of signals able to be handled in a signal extraction processing circuit is exceeded. In this case, there is a problem that correct information (signal) cannot be extracted.
In recent years, because of further capacity enlargement, there has been a tendency to record the information in a region finer than ever with a high density and to regenerate the information by the irradiation with light more intense than ever. In this case, the surge of the DC component of the regenerated signal tends to increase further in proportion to the intensity of the irradiation light.
A conventional method for solving the problem comprises: sorting the components which possibly cause the phase deviations between both P, S polarized components from the components constituting the optical system; or combining the phase deviation directions of a plurality of components causing the phase deviations in the optical system in order to mutually compensate for the phase deviations in the entire optical system.
However, in this method, the process of sorting the components or combining the directions is required, it is difficult to save costs, and the necessity of radiating further intense light to read the information cannot be handled.
Moreover, instead of the above-described method of sorting the components or selecting the combination, a method of disposing a phase plate immediately before the Wollaston prism on the optical path of the reflected light from the optical disk is proposed. This phase plate is an optical component which is disposed and inclined with respect to an optical axis, so that the thickness of the optical axis direction of the phase plate is changed. In accordance with the change of the thickness, that is, an inclination angle, the phase between both P, S polarized components can be adjusted. By disposing the phase plate and adjusting the inclination in each optical system, product dispersions are suppressed while the surge of the regenerated signal DC component can be controlled to provide a low level in any product.
However, in the above-described method of disposing the phase plate and adjusting the inclination angle to adjust the phase deviation between both P, S polarized components in the optical system, when the phase plate inclination is adjusted to adjust the phase deviation, the optical path ahead of the phase plate changes by the light refraction action of the phase plate. It cannot be monitored simply by changing the phase plate inclination whether or not the phase deviation is compensated. Additionally, it cannot be monitored whether or not the phase deviation is compensated until the position of the light receiving element or the like starts to be readjusted with the adjustment of the phase plate inclination. When the phase compensation Is insufficient, a process of readjusting the phase plate inclination and further readjusting the position of the light receiving element or the like to perform the monitoring needs to be repeated. A precise adjustment is abandoned, or the cost increase for the adjusting operation is abandoned and the above-described operation needs to be repeated many times as occasion demands.
In consideration of the above-described situations, an object of the present invention is to provide an optical storage apparatus including a constitution which can easily compensate for the phase deviation by an optical system between both P, S polarized components of the light subjected to a polarized state change in a storage medium, and a phase compensation amount adjustment method in the optical storage apparatus.
To attain the above-described object, according to the present invention, there is provided an optical storage apparatus in which by radiating light to a storage medium with information stored therein and receiving the light subjected to a polarized state change in the storage medium, the information stored in the storage medium is read based on the polarized state change.
The optical storage apparatus comprises a phase plate disposed in an optical path in which the light reciprocates so that a forward light emitted from a light source is radiated to the storage medium and a backward light subjected to the polarized state change in the storage medium reaches a light receiving element. The phase plate is disposed to compensate for a phase deviation between both P, S polarized components of the backward light.
In the constitution of the optical storage apparatus of the present invention, the light reciprocates by the phase plate. Therefore, when a phase plate inclination is adjusted, the optical path of the light which passes by the phase plate only once changes, but with respect to the light which again passes by the phase plate in a reverse direction, the change of the optical path is canceled. Therefore, by reciprocating the phase plate, even when the phase plate inclination is adjusted, the position of the light receiving element disposed after the phase plate does not need to be adjusted, the degree of phase compensation by the adjustment of the phase plate inclination can immediately be monitored, the adjustment of the phase plate inclination is remarkably facilitated, and a precise adjustment can be performed. Additionally, since the adjusting operation efficiency is high, cost reduction can be realized.
Here, when the light reciprocates by the phase plate, that is, passes by the phase plate twice, the phase plate compensates for the phase deviation between both P, S polarized components of the backward light. Therefore, one passage of the two passages needs to be performed by the backward light, but the other passage may be performed by the forward light. Alternatively, only the backward light may reciprocate by the phase plate twice.
As the optical storage apparatus of the present invention, concretely, the following modes can be employed. Specifically, in the optical storage apparatus, by radiating light to a storage medium with information stored therein and receiving the light subjected to a polarized state change by reflection by the storage medium, the information stored in the storage medium is read based on the polarized state change.
The optical storage apparatus comprises:
a light source for emitting the light to be radiated to the storage medium;
a light receiving element for receiving the light reflected by the storage medium;
a forward path optical system for guiding the light emitted from the light source to the storage medium via a predetermined forward path;
a backward path optical system for guiding the light reflected by the storage medium to the light receiving element via a backward path which traces the forward path in a reverse direction midway after the reflection by the storage medium, leaves the forward path midway and reaches the light receiving element; and
a phase plate, disposed on an optical path common to the forward path and the backward path, for compensating for a phase deviation between both P, S polarized components of the light tracing the backward path.
In this case, since the phase plate is disposed on the optical path common to the forward and backward paths, the light emitted from the light source passes by the phase plate once in the forward path, passes once also in the backward path from the reverse direction, and passes twice in total with reciprocation. Even when the phase plate inclination is adjusted, the position of the light receiving element or the like does not need to be readjusted. By monitoring the signal obtained by the light receiving element, and adjusting the phase plate inclination, the phase deviation can precisely and easily be adjusted.
Here, by rotating the phase plate around an axis having a direction different from that of the optical axis passed through the phase plate, the mounting angle of the phase plate to the optical axis is adjusted.
Moreover, when the phase plate is disposed obliquely at a predetermined angle to the optical axis passed through the phase plate, the phase compensation amount between both P, S polarized components preferably becomes zero.
The light passing by the phase plate is slightly reflected by the surface or the back surface of the phase plate. When this reflected light advances along the optical axis, the light possibly forms a noise component. Therefore, to shift the reflected light from the optical axis, the adjustment is preferably completed while the phase plate is disposed obliquely. By designing the phase plate so that the phase compensation amount becomes zero when the phase plate is disposed obliquely at the predetermined angle and so that a sufficient phase compensation width is provided, in any apparatus, the adjustment can be completed while the phase plate is constantly inclined, and the light reflected by the surface and back surface of the phase plate can be prevented from forming the noise light.
Moreover, in the optical storage apparatus of the present invention in which the phase plate is disposed in the optical path common to the forward and backward paths, the optical system formed by combining forward and backward optical systems is separately constituted of a fixed optical section fixed to a predetermined base and including the light source and the light receiving element, and a movable optical section moving with respect to the base in accordance with the access position of the storage medium and including only the optical path common to the forward and backward paths. The phase plate is preferably disposed on the portion of the fixed optical section via which the light is emitted to the movable optical section from the fixed optical section and upon which the light emitted to the fixed optical section from the movable optical section is incident.
When the phase plate is disposed in the above-described position of the fixed optical section, that is, in the position where the light reciprocates from and to the movable optical section, the position is held between the fixed optical section and the movable optical section with relatively a lot of space. Therefore, different from the optical system provided with no phase plate, the phase plate can be disposed without enlarging the entire size of the optical system or while minimizing the enlargement degree.
Here, the phase plate is disposed in the fixed optical section, but additionally a second phase plate for compensating for the phase deviation between both P, S polarized components of the light tracing the backward path is preferably disposed in the portion of the movable optical section upon which the light emitted to the movable optical section from the fixed optical section is incident and via which the light is emitted to the fixed optical section from the movable optical section.
In the adjustment of the phase plate both for the fixed optical section and movable optical section, for example, even when the fixed optical section and movable optical sections are separately assembled, the adjustment for the phase compensation can be performed separately on the fixed optical section and the movable optical section. Moreover, to provide the movable optical section with the phase plate, when the phase plate is disposed in the position between the fixed optical section and the movable optical section, the enlargement of the apparatus by disposing the phase plate can be avoided.
Moreover, when the phase plate is disposed both in the fixed optical section and movable optical section, the phase plate disposed in the fixed optical section and the second phase plate disposed in the movable optical section are preferably disposed in inclined states in opposite directions with respect to the optical axis.
In this constitution, the optical path deviating because of the obliquely disposed phase plate in the fixed optical section can be moved close to the original optical path by the second phase plate disposed in the movable optical section. Additionally, even if the optical path deviates in the movable optical section because of the inclination of the phase plate or the second phase plate for the phase compensation, the optical path deviation can be minimized.
Moreover, to attain the above-described object, according to the present invention, there is provided a phase compensation amount adjustment method for an optical storage apparatus in which by radiating light to a storage medium with information stored therein and receiving the light subjected to a polarized state change by reflection by the storage medium, the information stored in the storage medium is read based on the polarized state change. The optical storage apparatus comprises: a light source for emitting the light to be radiated to the storage medium; a light receiving element for receiving the light reflected by the storage medium; a forward path optical system for guiding the light emitted from the light source to the storage medium via a predetermined forward path; a backward path optical system for guiding the light reflected by the storage medium to the light receiving element via a backward path which traces the forward path in a reverse direction midway after the reflection by the storage medium, leaves the forward path midway and reaches the light receiving element; and a phase plate, disposed on an optical path common to the forward path and the backward path, for compensating for a phase deviation between both P, S polarized components of the light tracing the backward path.
The method of adjusting a phase compensation amount by the phase plate in the optical storage apparatus comprises: using a predetermined reference storage medium as the storage medium; monitoring the signal obtained by the light receiving element; and adjusting the inclination angle of the phase plate to the optical axis to minimize the phase deviation between both P, S polarized components of the light tracing the backward path.
According to the phase compensation amount adjustment method, by preparing as the storage medium the predetermined reference storage medium, typically the storage medium in which there is no change other than the change of the polarized state based on only the stored information, or the other changes are minimized, and accessing the reference storage medium, the phase compensation amount is adjusted. By performing this adjustment to minimize the phase deviation, the phase deviation attributed to the optical system can be compensated separately from the phase deviation in the storage medium.
Moreover, according to the present invention, there is provided a second phase compensation amount adjustment method for an optical storage apparatus in which by radiating light to a storage medium with information stored therein and receiving the light subjected to a polarized state change by reflection by the storage medium, the information stored in the storage medium is read based on the polarized state change. The optical storage apparatus comprises: a light source for emitting the light to be radiated to the storage medium; a light receiving element for receiving the light reflected by the storage medium; a forward path optical system for guiding the light emitted from the light source to the storage medium via a predetermined forward path; a backward path optical system for guiding the light reflected by the storage medium to the light receiving element via a backward path which traces the forward path in a reverse direction midway after the reflection by the storage medium, leaves the forward path midway and reaches the light receiving element; and a phase plate, disposed on an optical path common to the forward path and the backward path, for compensating for a phase deviation between both P, S polarized components of the light tracing the backward path. The phase compensation amount by the phase plate in the optical storage apparatus is adjusted in the method.
The method of adjusting a phase compensation amount by the phase plate in the optical storage apparatus comprises: preparing an adjusting light source for emitting a light imitating the light reflected by the storage medium instead of the above-described light source and storage medium and further preparing an adjusting light receiving element instead of the above-described light receiving element; receiving the imitating light emitted from the adjusting light source by the adjusting light receiving element via at least a part of the backward path including the phase plate; monitoring the signal obtained by the adjusting light receiving element; and adjusting the inclination angle of the phase plate to the optical axis to minimize the phase deviation between both P, S polarized components of the imitating light.
In this case, since the adjusting light source and adjusting light receiving element are prepared, the adjustment for the phase compensation can be performed on a part of the optical system constituting the optical storage apparatus, for example, the above-described fixed optical section.